Current microprocessor-based overcurrent relays operate on standard time-overcurrent trip characteristics. Those standards evolved as the technology of relays progressed from the initial electro-mechanical relays to solid state relays to the current microprocessor-based relays. When overcurrent relays were first developed they employed electro-mechanical switching mechanisms to provide the required overcurrent protection. Such electro-mechanical switching mechanisms, such as induction-disk type relays, exhibited certain time response characteristics. Accordingly, as the electro-mechanical relays became entrenched in the industry, their characteristic time-overcurrent response characteristics became standardized, resulting in the development of generally accepted families of overcurrent curves. Subsequently, as overcurrent relay development progressed to solid state devices, the solid state devices were designed to emulate standard electro-mechanical like overcurrent curves. These standard overcurrent curves were likewise emulated by the microprocessor-based relays that followed the solid state relays.
The solid state devices that developed used a series of linear segments to emulate the non-linear characteristics of the electro-mechanical relays. U.S. Pat. No. 4,017,766 (Vercellotti et al.), issued in 1977, discloses a solid state overcurrent relay that purportedly emulates the standard overcurrent curves via a series of successive linear time-overcurrent segments. By concatenating the linear segments Vercellotti emulated the non-linear electromechanical overcurrent relay characteristics. In this way, all the benefits of solid-state devices were brought to bear on the relay arts while accepted standard performance characteristics were maintained.
Eventually, the overcurrent relay industry adopted microprocessor-based mechanisms that emulated the earlier solid state devices, but with the added flexibility gained by a microprocessor. For example, the microprocessor-based relay overcurrent response curves could be quickly and relatively easily changed by reprogramming the relay. That is, microprocessor-based relays employ linear segments to emulate non-linear time-overcurrent characteristics. However, the linear segments could be rather easily changed to provide different time-overcurrent characteristics. For example, by simply changing data points (i.e., line segmented points), a previously installed relay time-overcurrent characteristic is altered to accommodate changes in the system in which it is employed. As a result, the useful life of a relay was extended because it could adapt to changing characteristics of the circuit.
Eventually, ANSI standards emerged that provided a standard time-overcurrent equation representative of the time-overcurrent response curves used by microprocessor-based overcurrent relays. The current proposed ANSI standard equation, per Draft ANSI standard C37.112, is defined below: ##EQU1## where: t is the trip time in seconds;
M is a current ratio that is equal to I divided by I.sub.P ; PA1 I is equal to the expected input current; PA1 I.sub.p is equal to the pickup set point current (i.e., the relay current set point as is well know to those of ordinary skill in the art); PA1 A is selected for the desired curve slope characteristic and must be greater than zero; PA1 P is selected for the desired curve characteristic and must also be greater than zero; and PA1 B is selected for the desired time to trigger offset. PA1 M is equal to I divided by I.sub.P ; PA1 I is equal to the input current; PA1 I.sub.P is equal to the pickup set point current; PA1 A is selected for the desired curve characteristic and must be greater than zero; PA1 P is selected for the desired curve characteristic and must also be greater than zero; and PA1 B is selected for the desired curve offset.
Notably, the ANSI standard is asymptotic to the time axis. That is, as the value of M.sup.P approaches one, t approaches infinity. Many applications find this to be an undesirable characteristic. Accordingly, the ANSI equation has been modified to define a value of t when M.sup.P is equal to one. Many microprocessor-based relays employ the modified ANSI equation of the form: ##EQU2## where C is less than or equal to one and the other variables are the same as in equation (1). This equation is often referred to as the modified ANSI standard.
In some conventional microprocessor-based relays, the values for the coefficients A, B, C and P in equations (1) and (2) are provided by the user to a computer which then computes a set of data points representing linear line segments representative of the desired non-linear overcurrent curve. Those line segments are then communicated to the microprocessor-based relay.
The present inventor has recognized that the prior art systems require a user to know the coefficients representative of a particular non-linear curve; however, the coefficients are difficult to derive from a graphical representation. In fact, the inventor has recognized that the user may not have ready access to the coefficients. Thus, there is a need for a system and method for supplying overcurrent curve data to an overcurrent relay that overcomes the drawbacks of the prior art.